Electroplating bath containing cuprous thiocyanate and cyanide and process of use



United States Patent 3,179,577 ELECTROPLATING BATH CONTAINING CU- PROUS THIOCYANATE AND CYANIDE AND PROCESS OF USE Elbert H. Hadley, Carbondale, Ill., assignor to Southern Illinois University Foundation, Carbondale, 11]., a corporation of Illinois No Drawing. Filed Jan. 10, 1962, Ser. No. 165,302 14 Claims. (Cl. 204-52) This invention relates to a plating bath and process and more particularly to a new and improved plating bath containing cuprous thiocyanate and an alkali metal cyanide as the essential reagents.

Briefly, the invention relates to a plating bath for the electrodeposition of copper comprising a solution containing 4.05 to 28.35 oz. per gallon of cuprous thiocyanate and an amount of alkali metal cyanide which is equivalent to a sodium cyanide to cuprous thiocyanate molar ratio of from 2.921 to 3.1:1. The invention also inlcudes the process of utilizing such bath for the electrodeposition of copper.

Among the several objects of the invention may be electroplating copper; the provision of such a bath and process which permits copper deposits of greater thickness to be obtained while retaining a higher degree of smoothness than was heretofore possible with deposits of equivalent thickness; the provision of a bath and process of the class described which gives satisfactory results over a wide range of current densities and temperatures; the provision of a bath and process of this character which yield deposits of satisfactory brightness; and the provision of such a bath and process which give reliable results. Other objects and features will be in part apparent and in part pointed out hereinafter.

The invention accordingly comprises the products and methods hereinafter described, the scope of the invention being indicated in the following claims.

As is known, conventional methods of electroplating copper are somewhat limited in usefulness in that the thickness of the deposits obtainable and which have the requisite smoothness does not ordinarily exceed 0.00075- 0.001". While deposits having a thickness in excess of these limits may be produced, the smoothness of such deposits is such that a butting operation is usually required. Therefore, existing copper plating methods do not permit deposits of greater thicknesses (i.e., on the order of 0.002" and up) to be obtained without sacrificing the desired smoothness. For many applications, this factor is important since deposits of greater thickness are desirable because they afford greater resistance to corrosion.

In accordance with the method and bath of the present invention, it has now been found that deposits of greater thickness (e.g., 0002-0005") may be obtained while retaining a smoothness of such quality that no butting operation is normally required. While the smoothness of the deposits obtained through the practice of the present invention, as in the case of deposits obtained through the use of conventional plating methods and baths, tends to deteriorate as the thickness of such deposits increases, this deterioration does not begin at as low a deposit thickness as in the case of previously used plating methods. Accordingly, the present invention makes it possible to obtain deposits of greater thickness which exhibit the requisite smoothness so that no bufiing operation is required.

Furthermore, the plating method and bath of the present invention permit the electroplating of copper to be carried out over a wide range of current densities and at cathode and anode efliciencies of substantially 100%.

noted the porvision of an improved bath and process for Also, the deposits produced in accordance with the present invention possess a satisfactory brightness when compared to deposits obtained from conventional baths and plating methods.

The above-stated advantages are attained through use of a plating bath containing cuprous thiocyanate (CuSCN, also known as cuprous sulfocyanide) and an alkali metal cyanide, preferably sodium cyanide, as the essential components. In general, the concentration of the cuprous thiocyanate in the bath can be varied over a wide range.

Broadly, the bath may contain from approximately 4.05 to 28.35 oz. per gallon (0.25 to 1.75 molar) of cuprous thiocyanate. Preferably, the bath contains from approximately 8.1 to 24.3 oz. per gallon (0.5 to 1.5 molar) or as a single preferred concentration 16.2 oz. per gallon (1.0 molar) of cuprous thiocyanate. The concentration of alkali metal cyanide in the bath can also be varied, but the important factor is the ratio of sodium cyanide to cuprous thiocyanate. Thus, I have found that the bath should include an amount of alkali metal cyanide, preferably sodium cyanide, Which is equivalent to a sodium cyanide to cuprous thiocyanate molar ratio of from approximately 2.9:1 to approximately 3.1:1. Preferably, an amount of alkali metal cyanide is employed which is equivalent to a sodium cyanide to cuprous thiocyanate molar ratio of 3:1. Thus, based on the above-stated preferred range for cuprous thiocyanate, the plating bath of the invention should contain 9.65 to 28.95 oz. per gallon of sodium cyanide or preferably 19.30 oz. per gallon of sodium cyanide. More broadly, the bath may contain from approximately 4.83 to 33.78 oz. per gallon of sodium cyanide. It will be understood that other alkali metal cyanides such as potassium cyanide may be used in place of sodium cyanide.

The addition of sodium hydroxide to the bath is beneficial to decrease the rate of decomposition of sodium cyanide and increase the conductivity of the plating bath. For example, if desired, 1-2 oz. per gallon (0.1875 to 0.375 molar), preferably 1 oz. per gallon, of sodium hydroxide may be included in the bath. It will be understood that other alkali metal hydroxides such. as potassium hydroxide may also be employed. p

In order to obtain the best results, it has been found that the plating bath of the invention should be operated hot, i.e., from a temperature of about 607 C. up to the boiling point of the bath. As a practical matter, the maximum temperature at which the bath may be operated is approximately 98 C.

With regard to cathode current density, it'has been found that any desired cathode current density may be used up to 100 or amperes per square foot. Current densities within this range permit operation at cathode efficiencies of substantially 100%, the particular current density at which a cathode efficiency of 100% is effected being dependent upon the other variable factors such as temperature, ratio of sodium cyanide to cuprous thiocyanate, etc. If a current density is used .at which the cathode efliciency is less than 100%, then any further increase in current density is accompanied by a decrease in cathode efiiciency. At any one current density, the decrease in efficiency is greater in more dilute baths and at lower bath temperatures.

As to anode current density, any value may be used at which anode etiicieney remains at 100%. Thus, anode current densities of from 10 to 200 amperes per square foot may be used, the particular current density at which 100% anode efficiency is achieved being dependent upon the conditions employed, i.e., temperature, molar ratio of reagents, etc.

It will be understood thatvarious addition agents such.

as are conventionally used in copper plating baths may be includedin the novelbaths of my invention to improve the quality (i.e., smoothness or brightness or both) of the deposits obtained.

The following examples illustrate the invention:

Example 1 A series of electroplating baths was prepared as follows:

Molar Concen- Molar Concentration of Cutration of S- prous Thioeyadium Cyanide nate (CuSCN) (NaCN) Bath No.

Anode efficiencies, cathode efficiencies, quality and thickness of deposits, conductance, anode and cathode current densities and pH measurements were determined for each of the above baths.

In carrying out the measurements, the baths were contained in a welded iron tank 12" x x 10". In all of the eiliciency measurements, the anode or cathode was agitated linearally at approximately 96" per minute while the correspondingefliciency was measured. The amount of current was measured in each case by a silver cyanidepotassium cyanide coulometer connected in series with the plating bath.

The cathodes employed were approximately 2" x 3.5" (0.1 sq. ft.) or half that size (0.05 sq. ft.), and were made from '28 gauge galvanized iron from which the zinc was removed with acid. Preliminary to the actual efiiciency measurement, the unbuffed, cleaned steel cathode was given acopper strike, washed thoroughly, dried and reweighed. The actual efliciency was calculated from the weight of copper deposited as compared with the amount which should have been deposited, the latter being calculated from the weight of silver having been deposited in the silver coul'ometer.

Electrolytic, copper anodes were used, andthe technique used in measuring the anode efiiciency was the same as that for the cathode efiiciency except that the efiiciency was calculated from the weight of copper dissolved.

Measurements were made for each bath at three different temperatures, namely, 60 C., 80 C. and 98 C.

Conductance measurements were made using a vacuum tube oscillator as an A.C. source'of 1000 cycles per second, a resistancebox, slide wire and end coils, earphones,

and a Leeds and Northrup N0. 4915 cell for high conductance liquids. The electrodes were platinized, 0.1 N potassium chloride. was used to determine the cell constant and measurements were made on each bath solution at temperatures of 25 C., 60 C., 80 C. and 98 C.

Measurements of pH were made with a Beckman A.C.

type pH meter using a Beckman high alkalinity electrode, 7

The deposits obtained varied in thickness from 0.002" to 0.005", thus being from two to five times greater than the thickness of deposits ordinarily plated by conventional plating methods. Under the optimum conditions stated below, the deposits of 0.002" and over were exceptionally smooth in comparison with other deposits of t thiocyanate ratio approached 3:1, but there was no further increase in smoothness at higher ratios. At the lower ratios, smoothness was increased at higher concentrations of the two reagents in the bath. Smoothness was unaffected by temperature and by cathode current density.

It was found that cathode etliciency tends to increase with higher temperatures, lower cathode current density, higher concentrations of cuprous thiocyanate'and lower sodium cyanide to cuprous thiocyanate ratios. Due to polarization, anode current efficiency tended to decrease more readily to values of less than 100% at lower molar concentrations of the basic reagents, at lower temperatures and at higher anode current densities (i.e., in excess of -200 amps/ft. under optimum conditions).

The brightness of the deposits obtained was equivalent to the brightness of deposits obtained from conventional copper plating baths under similar conditions. The

brightness of the deposits was somewhat better at lower temperatures if the sodium cyanide concentration was also low, and at higher ratios of sodium cyanide to cuprous thiocyanate, but was apparently unaffected by the molar concentrations of the bath reagents and by cathode current density.

Cathode efiiciencies of substantially 100% were achieved at'cathode current densities ranging from about 25 to about 150 amperes per square toot. Anode efiiciencies of substantially 100% were achieved at anode current densities ranging from about 10 to about 200 amperes per square foot.

Conductance measurements varied from about 5X10" to 22x10" mhos for the baths'having the lower concentration of cuprous thiocyanate and from about 15 10 to 43x10- mhos for-the baths having the higher concentration of cuprous thiocyanate. It was found that the conductance increased at elevated temperatures, the increase per degree becoming somewhat larger as the sodium cyanide concentration increased.

The pH values for the A series of baths ranged from 9.4 to 11.6 and for the E series from 9.6 to 10.0. Thus,

the basicity of these solutions increased as the amount of sodium cyanide was increased.

Based on these tests, the optimum operative conditions are as follows:

(5) Anode current density -any value at which the anode efliciency remains at 100%, i.e., from about 10 amps] ft. to 200 amps./ft.

, Example 2 A series of electroplating baths Was prepared as follows:

Molar Con- Molar Con- Molar Con- Bath N0. centration centration centration of CuSCN 0f NaCN of NaOH The procedure described in Example 1 was followed, and anode efficiencies, cathode efiiciencies, quality and thickness of deposit, conductance, anode and cathode current. densities and pH measurements were determined for each of the baths. Measurements were made for each bath at three different temperatures, namely, 60 C., 80 C. and 98 C.

The addition of sodium hydroxide appeared to have little or no effect on the maximum cathode current density at which the cathode efficiency remained at 100%. However, whenever the cathode efiiciency falls below 100%, the addition of sodium hydroxide was found to have a beneficial effect in that the cathode eificiency is higher in the presence of sodium hydroxide than when no sodium hydroxide is present. The lower the cathode efficiency, the greater is the beneficial effect of adding sodium hydroxide.

Cathode efficiencies of substantially 100% were achieved at cathode current densities ranging from about 20 to about 200 amperes per square foot. Anode efliciencies of substantially 100% were achieved at anode current densities ranging from about 5 to about 200 amperes per square foot.

The optimum sodium hydroxide concentration was found to be approximately 0.1875 molar. Increasing the sodium hydroxide concentration from 0.1875 molar to 0.375 molar does not cause any appreciable further increase in beneficial effect. Further, there was no marked difference in the beneficial effect of sodium hydroxide on cathode efficiency when (1) the cuprous thiocyanate concentration was increased from 0.25 molar to 1.25 molar or (2) when the temperature was increased from 60 C. to 80 C. or from 80 C. to 98 C.

Under the conditions studied, the addition of sodium hydroxide lowers the maximum anode current density at which the anode efiiciency remains at 100%. The addition of sodium hydroxide has a greater deleterious effect on anode efficiency at higher temperatures and higher concentrations of sodium cyanide. However, increasing the sodium hydroxide concentration from 0.1875 molar to 0.375 molar does not appreciably increase the deleterious efiect of the sodium hydroxide on anode efiiciency. When the ratio of sodium cyanide to cuprous thiocyanate in the plating bath was appreciably less than 3:1, the solution turned blue and a black precipitate was formed when the bath was allowed to stand overnight. When the ratio of sodium cyanide to cuprous thiocyanate was equal to or slightly greater than 3:1, the deposits were in general smooth and uniform in color.

The quality and thickness of the deposits from the above baths containing sodium hydroxide were generally equivalent to the quality and thickness of the deposits produced in Example 1 from plating baths containing no sodium hydroxide.

Example 3 A series of electroplating baths was prepared as follows:

Molar Con- Molar Con- Molar Con- Buth No. centration centration centration of CuSCN of NaCN of NaOH The bath designated G-l represents a saturated solution at room temperature.

The procedure described in Example 1 was followed and anode efficiencies, cathode efi'iciencies, quality and thickness of deposits, conductance, anode and cathode current densities and pH measurements were determined for each of the baths. Measurements were made for each bath at three different temperatures, namely, 60 C., 80 C. and 98 C.

It was found, as in the tests described in Examples 1 and 2, that as the concentration of cuprous thiocyanate in the plating bath was increased, the cathode efficiency.

at any selected current density was higher. Also, as the temperature was increased, the cathode efiiciency was higher. While the cathode efficiency is higher at higher concentrations, the percentage increase is less at higher temperatures than at higher concentrations.

It was also found, as in Examples 1 and 2, that asthe concentration of the plating bath was increased or as the temperature increased, the maximum anode current density at which anode efficiencies remained at was also increased.

Cathode efficiencies of substantially 100% were achieved at cathode current densities ranging from about 50 to about amperes per square foot. Anode efficiencies of substantially 100% were achieved at anode current densities ranging from about 50 to about 100 amperes per square foot.

The quality and thickness of the deposits produced was generally comparable to the quality and thickness of the deposits obtained in Examples 1 and 2. Smooth and uniform deposits were obtained at each of the temperatures and concentrations tested, but the deposits made at higher current densities were not quite as uniform and smooth as those made at lower current densities. Changes in temperature and in concentration per se did not alter the quality of the deposit.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above methods and products without departing from the scope of the invention, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A plating bath for the electrodeposition of copper comprising a solution containing approximately 4.05 to 28.35 oz. per gallon of cuprous thiocyanate and an amount of alkali metal cyanide which is equivalent to a sodium cyanide to cuprous thiocyanate molar ratio of from 2.9:1 to 3.121.

2. A plating bath for the electrodeposition of copper comprising a soltion containing approximately 8.1 to 24.3 oz. per gallon of cuprous thiocyanate and an amount of alkali metal cyanide which is equivalent to a sodium cyanide to cuprous thiocyanate molar ratio of from 2.9:1 to 3.1:1.

3. A plating bath for the electrodeposition of copper comprising a solution containing approximately 16.2 oz. per gallon of cuprous thiocyanate and approximately 19.30 oz. per gallon of sodium cyanide.

4. A plating bath for the electrodeposition of copper comprising a solution containing approximately 8.1 to 24.3 oz. per gallon of cuprous thiocyanate, approximately l to 2 oz. per gallon of an alkali metal hydroxide and an amount of alkali metal cyanide which is equivalent to a sodium cyanide to cuprous thiocyanate molar ratio of from 2.9:1 to 3.1:1.

5. A plating bath for the electrodeposition of copper comprising a solution containing approximately 8.1 to 24.3 oz. per gallon of cuprous thiocyanate, approximately 1 to 2 oz. per gallon of sodium hydroxide and an amount of alkali metal cyanide which is equivalent: to a sodium cyanide to cuprous thiocyanate molar ratio of from 2.9:1 to 3.1: 1.

6. A plating bath for the electrodeposition of copper comprising a solution containing approximately 16.2 oz. per gallon of cuprous thiocyanate, approximately 1 oz. per gallon of sodium hydroxide and approximately 19.30 oz. per gallon of sodium cyanide.

7. The process for the electrodeposition of copper which comprises electrolyzing a solution containing ap proximately 4.05 to 28.35 oz. per gallon of cuprous thiocyanate and an amount of alkali metal cyanide which is equivalent to a sodium cyanide to cuprous thiocyanate molar ratio of from 2.9:1 to3.1 1.

8. The process for the electrodeposition of copper which comprises electrolyzing a solution containing approximately 8.1 to 24.3 oz. per gallon of cuprous thiocyanate and an amount of alkali metal cyanide which is equivalent to a sodium cyanide to cuprous thiocyanate molar ratio of from 2.9:1 to 3.1:1.

9. The process for the electrodeposition of copper which comprises electrolyzing a solution containing approximately 162 oz. per gallon of cuprous thiocyanate and approximately 19.30 oz. per gallon of sodium cyanide, said solution being maintained at a temperature greater than 70 C.

10. The process for the electrodeposition of copper which comprises electrolyzing a solution containing approximately 8.1 to 24.3 oz. per gallon of cuprous thiocyanate, approximately 1 to 2 oz. per gallon of sodium hydroxide and an amount of alkali metal cyanide which is equivalent to a sodium cyanide to cuprous thiocyanate molar ratio of from 2.9:1 to 3.1:1, said solution being maintained at a temperature greater than 70 C.

11. The process for the electrodeposition of copper which comprises electrolyzing a solution containing approximately 8.1 to 24.3 oz. per gallon of cuprous thiocyanate, approximately 1 to 2 oz. per gallon of sodium hydroxide and an amount of alkali metal cyanide which is equivalent to a sodium cyanide to cuprous thiocyanate said solution being maintained at a temperature of approximately 98 C.

13. A'plating bath for the electrodeposition of copper comprising a solution containing approximately 4.05 to 28.35 oz. per gallon of cuprous thiocyanate and an amount of alkali metal cyanide which is equivalent to a sodium cyanide to cuprous thiocyanate molar ratio of not less than approximately 2.9: 1.

14. The process for the electrodeposition of copper which comprises electrolyzing a solution containing approximately 4.05 to 28.35 oz. per gallon of cuprous thiocyanate and an amount of alkali metal cyanide which is equivalent to a sodium cyanide to cuprous thiocyanate molar ratio of not less than approximately 2.9:1.

6/42 Wernlund et a1. 204 52.1- 

1. A PLATING BATH FOR THE ELECTRODEPOSITION OF COPPER COMPRISING A SOLUTION CONTAINING APPROXIMATELY 4.05 TO 28.35 OZ. PER GALLON OF CUPROUS THIOCYANATE AND AN AMOUNT OF ALKALI METAL CYANIDE WHICH IS EQUIVALENT TO A SODIUM CYANIDE TO CUPROUS THIOCYANATE MOLAR RATIO OF FROM 2.9:1 TO 3.1:1. 